The present invention relates to the measurement of intra-ocular distances using an ultrasonic probe, specifically for measuring the axial (anterior to posterior) length of the eye and particularly to using an ultrasound probe for accurately measuring the eye prior to lens calculation for cataract surgery.
The use of high frequency ultrasound to determine the anterior to posterior dimensions of the eye is well known. The measured axial length of the eye (distance from the cornea to the retina) is used to calculate the lens implant power to be used in cataract surgery. Although optical methods for measuring the axial length of the eye have been described, ultrasound measurement is the preferred method used by ophthalmologists. The technology and technique for making ultrasound measurements in ophthalmology are disclosed in Coleman, D., Ultrasonography of the Eye and Orbit. Lea & Febiger (1977). To make such measurements, a hand-held ultrasound probe is placed in contact with the cornea of the eye. The probe includes a transducer and a separate internal fixation light which the patient is instructed to view. As the patient views the internal fixation light, high frequency ultrasonic waves are reflected off of the back of eye, and these reflections are processed by the transducer and converted to quantifiable distances.
A description of the type of transducer used in this device can be found in U.S. Pat. No. 4,213,464 to Katz et al and a description of the complete measuring system can be found in U.S. Pat. No. 4,154,114 Katz et al Other U.S. patents which describe ocular measurement devices include U.S. Pat. No. 4,508,121 to Myers, U.S. Pat. No. 4,564,018 to Hutchinson et al and U.S. Pat. No. 4,576,176 to Myers.
Axial length measurement of the eye, although simple in concept is difficult in practice because the eye cannot be held stationary during the exam. Furthermore, the eye is pliable and excessive pressure applied to the eye during the measurement process will compress the eye and create inaccurate results. Finally, the measurement takes place on a human, which even when cooperative, typically has gross head, neck and body movement during the procedure.
Precise axial length measurement of the eye with ultrasound requires the following three components: 1) proper alignment of the ultrasound probe along the visual axis, 2) a mechanism of gently positioning the ultrasound probe against the eye during measurement with the correct and constant pressure applied to the eye by the ultrasound probe tip, and 3) stationary positioning of the ultrasound probe relative to the patient's head and eye during the measurement process.
Any one of these three variables, if not controlled, can confound the measurement process and introduce major errors during lens calculation for cataract surgery. Despite excellent technician skill, axial length measurement has been confounded by these variables since its inception. Numerous techniques have been developed in an attempt to minimize these sources of potential error.
A central problem discovered when utilizing and evaluating prior art for axial length ultrasound measurement is that each solution typically focuses on minimizing one or two potentially confounding factors, leaving the other variables to chance. The 3 main variables will be discussed in turn, the prior art solutions will be reviewed and discussed, and then the approach to solving all three of the potentially confounding variables to axial length measurement will be revealed.
1) Misalignment of the Probe during measurement: Misalignment of the probe during ultrasound measurement causes gross errors in axial length measurement as a result of off-center and/or off-axis positioning. Either of these errors will lead to incorrect calculations of the lens power used for cataract surgery. To minimize this problem, most current ultrasound probe tips have an internal fixation light provided by a fiber optic system to assist with fixation as described in U.S. Pat. No. 4,934,370 to Campbell. These internal fixation lights and the process of using them have limitations in their ability to assure proper alignment during ultrasound measurement. For example, the Sonomed A-Scan 5500 Ultrasound, a popular ophthalmic ultrasound measuring device, gives the following instructions for assuring proper alignment of the probe with the visual axis. “Instruct the patient to look towards the red fixation light in the probe tip and visually align the probe along the patient's visual axis”. The fundamental limitation to this process is that the internal fixation point in the prior art is too large to adequately limit movement of the eye. As a result, patients cannot assist in maintaining axial alignment during measurement. Another fundamental problem is that the complex movements of the eye are extremely difficult to follow using a hand-held probe.
The first limitation of the internal fixation light provided in the prior art, U.S. Pat. No. 4,934,370 to Campbell, is the large size of the fixation light. In U.S. Pat. No. 4,934,370, pinhole optics are used to present a fixation target to the patient before and during contact of the probe with the eye to help maintain the patient's eye in alignment with the ultrasound waves emitted from the tip of the probe. This invention improves upon the previous process, where no fixation target was employed; however, the internal fixation system has limitations because of its large size. Campbell alludes to the large size of the fixation spot in his U.S. Pat. No. 4,934,370 in section 2 line 50 “as (the ultrasound probe tip) approaches, the point source increases in size. When the light source becomes SUFFICIENTLY LARGE, the fixation target becomes visible”.
“Sufficiently large” refers to the size of the fixation target presented to the patient once the ultrasound probe of the prior art makes contact with the surface of the eye. Because of the inherent size of the pinhole optics and light scatter that occurs due to the tear film, the apparent size of a 1 mm fixation spot (a common size of pinhole used in pinhole optics) enlarges upon contact with the eye to subtend approximately 15 degrees in the central vision. Using the reduced schematic of the Gullstrand eye (American Academy of Ophthalmology Basic Science Book 3 Optics, 2002), it can be predicted that a 1 mm fixation spot in contact with the cornea is at least 8 times larger than the anatomical fixation capability of eye. A 1 mm fixation spot located at the cornea is sufficiently large to allow the patient to look approximately 15 Prism Diopters (7.5 degrees), in any direction and still have the “fixation target” overlying the central fixation point of the retina. Because of the large fixation spot, after the probe contacts the eye, the patient can have significant misalignment of the eye and still be looking at the large fixation point. Because of the large apparent size of a 1 mm fixation spot after it contacts the cornea, the current standard in the industry provides a rough guide for fixation, but does not precisely assure proper alignment of the ultrasound probe with the visual axis.
The alignment implications of having a large fixation spot is that the patient can maintain fixation of the large spot even when the tip of the probe is off center or the entire probe is misaligned and off the visual axis. The large spot size combined with the inherent difficulty of aligning the probe along the visual axis of a “live” eye leads to confounding errors during measurement. Visually aligning the probe along the visual axis while holding the probe stationary against the eye with the correct amount of pressure is quite difficult. Complex alignment relationships must be maintained between the probe and the eye at all times. All ultrasound probes have a tip and a tail. The tip emits the ultrasound signal and receives acoustic echoes from the internal ocular structures. The tail has a wire attached which is connected to the main computer. The tip-to-tail axis of the probe corresponds to the direction of the projected ultrasound beam emitted from the tip of the probe. For accurate results, precise alignment of both the tip and the tail of the probe must be maintained at all times. The two main categories of probe misalignment include “off center” problems, where the tip of the probe is not centered over the anatomical center of the cornea, and “off axis” problems, where the axis of the ultrasound probe does not correspond to the visual axis of the eye being measured. Either of these positioning errors will give inaccurate readings.
Off Center Problems: The first aspect of achieving the correct ultrasound probe alignment is locating and maintaining the correct contact point between the tip of the probe and the topographical center of the cornea. The desired axis for measuring the axial length of the eye is along the visual axis. This relationship is most accurately described by the axis of Fick. As described in the American Academy of Ophthalmology Basic Science series 2002, Book 6, 2002, the “Y” axis of Fick is a sagittal axis passing through the center of the cornea, pupil and out through the posterior of the eye. To correctly place the tip of the probe on the center of the cornea, it must come in contact with the cornea at a spot that corresponds to the center of the pupil. While performing an ultrasound, the technician is typically positioned to the patient's side looking at the pupil from an oblique angle. Extrapolating the pupil's center from this location can be quite difficult. Typically, the patient's eye is wandering about throughout the measurement process because the fixation spot inside the prior art probe is too large to confine the movement of the eye as stated above. Since the cornea is a sphere, any off-center positioning of the probe tip will give an erroneously short measurement. In fact, short measurements are the most commonly encountered error seen in ultrasound axial measurements, in part because of the difficultly of visually maintaining the proper orientation of the probe on the moving cornea.
Off Axis Problems: The second aspect of achieving the correct ultrasound probe alignment is maintaining the proper relationship between the tip and the tail of the ultrasound probe. Once the tip of the probe is successfully placed on the geographic center of the cornea, the tail of the probe must be aligned with the tip in a way that the direction of the beam emitted from the tip of the probe is precisely aligned with the visual axis of the eye. Patients typically do not give feedback during this procedure, so there is no objective way of knowing if the ultrasound probe is correctly aligned. The fixation spot in the prior art is quite large, as described above, and can be seen at almost any angle of incidence with regard to the tip-to-tail alignment of the probe. Therefore, proper axial alignment cannot be objectively confirmed.
Axial alignment of the ultrasound probe, also known as the tip to tail alignment, is quite complex as it relates to a moving eye. To understand why it is so difficult to maintain this alignment, one must understand the geometry of the eye itself. The central axis of rotation of the eye is located at the center of the eye, not the cornea. This means that during measurement of the eye, if the patient inadvertently looks left 1 mm, the cornea moves 1 mm to the left while the retina moves 1 mm to the right. In this situation, the technician performing the ultrasound would need to quickly move the tip of the probe 1 mm to the left along an imaginary spherical meridian, while moving the tail of the probe 3-4 mm to the left, depending on the length of the probe. This would need to be done rapidly but without changing the force applied to the cornea by the probe tip. This extremely complex movement is very difficult to perform in a “live” eye that has the ability to move at a much higher rate of speed than the hand that is trying to follow it. In addition to the fine movements of the eye, the patient's head is also unstable and tends to move during the procedure.
Finally, most patients tend to have an avoidance response and their entire body moves away from the technician during the course of the measuring process thereby altering critical landmarks and making it quite difficult to maintain proper alignment.
In summary, due to the large fixation spot size of the prior art, the patient can look 15 prism diopters (7.5 degrees) in any direction and still be looking directly at the fixation spot. The technician can move the probe “off center” in any direction and still have the fixation spot fully visible. The axial alignment of the probe (tip to tail alignment) can be significantly “off axis” yet the patient can still clearly see the fixation spot. Due to these limitations, the internal fixation spot presented in the prior art is not the ideal way to maintain fixation during axial length measurement of the eye with ultrasound. It is no surprise that difficulty abounds in attempting to provide consistently accurate ultrasound readings.
2) Excessive or inadequate force applied during measurement: Attempts have been made to dampen and modulate the pressure that is placed on the eye while taking axial length measurements with an ultrasound probe. One notable invention that attempts to accomplish this is the Hand-held Spring Loaded Probe, U.S. Pat. No. 4,930,512 to Henriksen et al The prior art focuses on applying constant pressure to the eye by way of a spring dampened hand-held probe that aids in avoiding compression of the eye by limiting the maximum force applied during measurement.
This apparatus works relatively well in practice; however, there are two limitations to the proposed mechanism.
The first limitation is that the device must be held in the technician's hand and the problems of “off-center” and “off-axis” alignment discussed above become a confounding problem. The device can be connected to a slit lamp for more precise control, but there is still no inherent way to assure alignment with the visual axis.
The second limitation is that the resistance applied by the spring to the tip of the probe is fixed at a certain amount. This means that if the intra-ocular pressure is less than this amount, the probe will compress the eye and cause short readings. This probe dampens excessive pressure in the plane directly to and away from the eye, but any tangential movement on the surface of the eye moves the inner barrel of the spring loaded probe against the side wall of the outer sleeve and may decrease the ease with which the two sleeves slide past one another causing the resistance applied to the tip of the device to increase, thereby compressing the cornea.
3) Stabilization of the probe during measurement: This problem has been addressed in prior art by two different approaches. The first approach involves mounting the ultrasound probe on a slit lamp and having the patient sit at the slit lamp during the measurement process. This approach increases stability, only to the extent that the patient remains in perfect alignment with the slit lamp. It does not address the compression of the cornea, nor does it address the axial alignment issue. The second approach involves using a water immersion bath which gives the probe some ability to move towards and away from the eye during the measurement without compressing the eye. This approach addresses the compression issue, since the intervening water bath prevents contact between the probe and the eye; however, it does not address the axial alignment issue. Furthermore, the water bath technique takes much longer to perform, involves laying the patient in a supine position, using a lid speculum and requires the use of a coupling agent (water bath) which can be messy. Due to limitations listed above, neither the slit lamp technique nor the water bath technique fully address all three limitations of ultrasound measurement of the eye, which include, 1) proper alignment of the ultrasound probe along the visual axis, 2) a mechanism of gently positioning the ultrasound probe against the eye during measurement with the correct and constant pressure applied to the eye by the ultrasound probe tip, and 3) stationary positioning of the ultrasound probe relative to the patient's head and eye during the measurement process.